There is a great deal of interest at the present time in optical logic elements not only because of their potential capability for performing fast logic operations but also because of the possibility they afford for construction of massively parallel computer architectures. It is contemplated that a single array of optical logic elements might contain at least 10.sup.6 logic gates which would function simultaneously. Several such arrays might be optically interconnected by a series of lenses thus permitting operation of more logic elements in a given time period than is presently contemplated for electronic logic elements.
Several types of optical logic elements have been developed. For example, highly nonlinear semiconductor materials such as InSb, InAs or GaAs may be used in optical bistable devices. See, for example, Applied Physics Letters, 42, pp. 131-133, Jan. 15, 1983. Use of such semiconductors in multiple quantum well (MQW) devices relying on absorption effects caused by excitons has also been demonstrated. One promising MQW device is termed the self-electro-optic effect device (SEED) and uses optically influenced electric fields to modulate the light beam. See, for example, Applied Physics Letters, 45, pp. 13-15, 1984. These elements may be termed single beam logic elements.
Yet another approach to optical logic elements uses a nonlinear Fabry-Perot etalon to form logic gates. See, for example, Applied Physics Letters, 44, pp. 172-174, Jan. 15, 1984. This technique uses, for example, two input beams and a probe beam with a nonlinear medium selected so that the absorption of a single input pulse changes the refractive index enough to shift the Fabry-Perot transmission peak near the probe beam wavelength by approximately one full width at half maximum. Of course, the peak returns to its initial wavelength after the medium relaxes. However, the probe transmission immediately after the input beams are incident on the etalon determines the output. Pulsed operation was also contemplated and even preferred. This type of logic element will be referred to as a dual beam device as the device distinguishes between two beams, in this case because they are at different wavelengths.
Similar work has described, for example, optical modulation by optical tuning of a Fabry-Perot cavity, but the potential for performing logic operations was not explicitly described. The transmission of a single beam through the cavity was modulated by a control beam which varied the refractive index of the cavity medium thereby changing the refractive index for the signal beam. See, for example, Applied Physics Letters, 34, pp. 511-514, Apr. 15, 1979.
Optical logic elements afford, at least theoretically, enormously enhanced switching capabilities as compared to electronic logic elements. Many optical logic elements, however, are subject to limitations that are not common in electronic logic elements. One such limitation relates to directionality. Optical elements accept light beam inputs from directions other than the preferred input side. Consequently, extraneous light beams may be accepted as inputs. As a result, the device operation is impaired. While lack of directionality is not objectionable in small arrays of widely spaced logic elements, closely packed logic elements in large arrays gives rise to extraneous beams that are inadvertently applied in a direction reverse to the information flow. As a result, false operation or a lowering of the noise immunity of the logic elements may be experienced. It is an object of the invention to provide improved optical logic elements which are asymmetric whereby data flow is optimized in one direction and inhibited in the reverse direction.